1. Field of the Invention
The present invention relates to a radio receiver comprising an antenna tuning circuit whose tuning frequency is improved.
2. Description of the Related Art
In general, when a radio receiver receives disturbance stations fud1, fud2 having a strong field intensity, in addition to an object station fd which the radio receiver desires to receive the stations fd, fud1, fud2 having frequencies different from one another by an equal extent (see FIG. 3(i)), the receiver generates various harmonic signals according to the non-linear element characteristics of an amplifying element, such as a transistor, of an RF amplifier thereof. Those harmonic signals may have frequencies corresponding to the addition or difference of the frequencies of object and disturbance stations fu, fud1, fud2.
If the frequency of such a harmonic signal is equal to that of the object station or the intermediate frequency, the harmonic signal may cause mutual modulation disturbance to the object station. FIG. 5 is a diagram showing a circuit structure of a conventional radio receiver which is less affected by mutual modulation disturbance because an antenna tuning circuit 1 is employed.
Referring to FIG. 5, a received RF signal is supplied to an antenna tuning circuit 1, which is then tuned to the object station and outputs a signal into an RF amplifier 2 for amplification. After amplification, the signal is then supplied to an RF tuning circuit 3, which is then tuned and outputs a signal into a mixing circuit 4. In the mixing circuit 4 , the signal is rendered to frequency conversion using a local oscillation signal supplied by a local oscillation circuit 5 and converted into an IF (intermediate frequency) signal. The IF signal is then output to an IF amplifier 6 for amplification, and subsequently to an FM detection circuit 7 for FM detection.
It should be noted that the mixing circuit 4 supplies an IF signal also to a field strength detection circuit 8. In the circuit 8, the peak level of the supplied IF signal is detected to thereby know the field intensity of the relevant received signal. The circuit 8 then supplies a signal based on the detection result to a dumping control circuit 9. The circuit 9 in turn detects whether or not the supplied signal is higher than a predetermined level, and outputs a signal based on the detection result to a dumping circuit 10 to thereby turn on/off the dumping circuit 10.
In the above procedure, if disturbance stations fud1, fud2 having high field intensity are also received together with an object station fd, the field strength detection circuit 8 outputs a higher than a predetermined level signal. Accordingly, the dumping control circuit 9 outputs a signal a. In response to this signal a, a current flows into PIN diodes 10a and 10b in the dumping circuit 10.
When a current flows in the PIN diodes 10a and 10b, the resistance values thereof become smaller, so that the receiving line is AC grounded via the PIN diode 10a, whereby the dumping circuit 10 is turned on (ON state).
As described above, since a received RF signal is partly grounded when the dumping circuit 10 is in an ON state, an output signal level from the antenna tuning circuit 1 is attenuated, so that the subsequent RF amplifier 2 receives a reduced level signal. As a result, a harmonic signal level is reduced, and mutual modulation disturbance is resultantly suppressed, the harmonic signal being generated based on object and disturbance stations according to the non-linear characteristics of elements in the RF amplifier 2.
In addition to the above, the antenna tuning circuit 1 is adjusted to have a tuning frequency so as to match the frequency of an object station (see the solid line in FIG. 3 (i)) by thus adjusting the characteristics of a coil 1a and a capacitor 1b and the capacity of a varicap diode 1c. Specifically, the capacity of a varicap diode 1c is adjusted according to the frequency of an object station in response to a control signal generated by a control circuit 11. With this adjustment, the antenna tuning circuit 1 is tuned to the frequency of an object station and those in the vicinity thereof.
However, when the dumping circuit 10 is once turned on and a DC blocking capacitor 10c is thereby connected in parallel to the capacitor 1b and the varicap diode 1c, the tuning frequency of the antenna tuning circuit 1 is caused to be shifted toward a lower frequency side (see the dot line in FIG. 3 (i) ) by an extent corresponding to the added capacity of the DC blocking capacitor 10c. In particular, if disturbance stations fud1, fus2 are present on the lower frequency side with respect to the object station fd (see FIG. 3 (i)), the level of a signal relative to an object station becomes lower than that of the disturbance fud1, fud2, when the dumping circuit 10 is turned on. Thereby, the level of a harmonic signal based on the disturbance stations fud1, fud2, is relatively increased. As a result, mutual modulation disturbance may become worse.